Frequency discriminator network



Aug. 5, 1952 D. MAcKl-:Y

FREQUENCY DISCRIMINATOR NETWORK Filed March 29. 1947 @aA/44a McKay. BYATTORNEY Patented Aug. 5, 1'952 2,606,250 FREQUENCY DisoiuinNAroRNETWORK Donald Mackey, Haddon Heights, N. J., assigner to RadioCorporation of America, a corporation ofDelavvare Application March 29,1947, Serial No. 738,056 3 Claims. (Cl. 17 8-44) My present inventionrelates generally to a novel and improved discrimination input circuitfor detectors of angle modulated carrier waves, and more particularly toan improved ratio detector circuit for deriving the modulation signalfrom a frequency modulated (FM) carrier wave without allowing(5o-existent amplitude modulation (AM) variations to result insubstantial detector output potentials.

It is an important obiect of my present invention to prov-ide afrequency discriminator input circuit for a detector of angle modulatelcarrier waves, the input circuit comprising cascaded resonant circuitsemploying a novel coupling` construction at the final circuit to providethevrequired pair of detector input voltages whose relative magnitudesare a-function of the angle modulation.

It is another important object of my invention to provide a ratiodetector for FM signals of the type disclosed by S. W. Seeley inapplication Serial No. 614.956, nled September '7, 1945, now Patent2,497,841 issued on February 14, 1950, the detector-being constructed topermit the use of cascaded resonant input circuits, iron cores 4beingemployed for adjusting the respective inductances of the input circuits,and an iron core being utilized to adjust the electrical center of thetapped final resonant circuit to provide a symmetricalresponse.

A specific object of my invention isto provide a novel method.- ofcoupling a pair of shielded resonant circuits whereby there aredeveloped at opposite sides of an electrical center of the second-circuitrespective high frequency voltages, each of the latter being theresultant of applied high frequency lvoltages in phase quadraturerelation.

More specinc objects of my invention will appear in the followingdetailed description of an embodiment, it being pointed out that. mylpresent balancedFM detector system, substantially immune toAlvleffects, has particular applicability to FM receiver construction.

Other-objects and features of the invention ivil'lbest be understood byreferenceto the -following description, taken in` connectionwith thedrawing, in which I have indicated digrammatically circuits whereby myinvention may be carried into effect. v

In the drawing:

Fig. 1 shows anembodiment of my present invention; 2 shows ka sectionthrough the input coil form of the rectiers; and

Fig. Sillustrates a modification of the circuit of Fig. 1.

Referring now to the accompanying drawing, wherein like referencenumerals in the figures denote similar circuit elements, there is shownin Fig. i a detector network of aniFfM receiver of the superheterodynetype.` rIlnedetector. input circuit is constructed in accordance withvmy presentinvention. While my invention is readily incorporated inanyform of receiverof waves, l prefer to explain its functioning .inYconnection lwith a superheterodyne receiver since such asystem is widelyused at thel-pres'ent-timfe. As previously explained, thepresentinvention is nel; resumed 12,0 reception uf F'M'Wevs' ws phasemodulation carrier waves could Akbe received aswell. An FM wave isprqducedat thellyi transmitter by deviating the carrier wave rela-A tiveto its mean frequency toy an yex, tent proportional to the amplitude ofthe modulation ,signal and independent of the frequency of the m duflatins Signal.' A PM wave.' differs in lievi. sa frequency deviationwhich increases ,with the frequency of the niodulat,ion signal. lThegeneric expression, angle modulation is, also, intended to include Vamodulated vwave of preferably, con: stant amplitude wherein theVnfiodulatmi is neither pure. FM nor pure PM, but containscomponentsresemblingvone or both of, them, `@and is,

therefore, a hybrid'fvmodulation.

:In the .present applicati@ itis assumed, by wa of specific example,anunciados" signed to ,operate in the broadcast, bandlpof 88-108megacycles (me), andfthat each-trans? mitter radiates an FM wave havinga frequency deviation up tov i775 ocycles-kc.) with respect fw thenormal transmitter flirquency- These are the, assigned frequency. vel:ues -Of the present lireadesi bend ,andere used. herein .merely by wwwillustrative The receiver may .include any desired form-,0i Signalcollector,y asv forexample a dipole The; Clleld FM signal waves may,heappli Converter for, reductlor10f-tl1e .rleen.. value without change0f the deviation.. cozweiter l may be, of, any Adesired construction.and is preferablrprcceded by one more .stages of selective radiofrequency amplincanon. Soitable signal selector circuits, usuallyemploying a Y variable condenser or adjustable inductor, are employedfor adjustment to receivesignals from a desired FM station. Thesignallselector cin.- cuits will, of course, Vpreferably tpe-,adjustedto resonate the Various adjustable selector circuits 3 to the center ormeans frequency of the desired FM station.

In a superheterodyne receiver, as is well known, the converter is fedwith oscillations from a local oscillator whose tank circuit includes anadjustable reactance device, usually a variable condenser or adjustableinductor. The latter is customarily adjusted concurrently with theaforesaid signal selector devices so that the tank circuit will be tunedto a local oscillation frequency diifering from the desired carrierfrequency by the operating intermediate frequency (I. F.). The selectivecircuits of, and preceding, the converter may on the other hand be ofthe xedly tuned type, if desired. The intermediate frequency7 is usuallychosen from a range of 2 to l mc., by way of example 10.7 mc. Anysuitable actuating mechanism may be used for operating the stationselecting devices. The oonverter may use the well-known pentagrid tube,or it may use separate oscillator and mixer tubes. These variouscircuits and circuit components are very well known to those skilled inthe art of radio communication, and need only be briefly referred to.

The I. F. amplifier network may embody one or more amplifier tubesselectively tuned to the operating I. F. value of 10.7 mc. Of course,all signal transmission circuits between the signal collector and thedemodulator or detector will be so constructed as to pass efliciently aband at least 150 kc. Wide. It is, also, usual to design the signaltransmission circuits to have a pass band of approximately 200 kc. inwidth to provide for reasonable tolerances, such as oscillator frequencydrift and the like. The transformer I feeding the final I. F. amplifiertube 2 has its primary and secondary circuits 3, 4 each tuned to theoperating I. F. value.

One of the reasons in the past for employing an amplitude limiter priorto the discriminator section (or FM translating network) of thedemodulator to reduce undesired AM effects on the carrier wave, was toavoid the necessity for critical tuning to the exact center, or carrier,frequency of a desired FM wave. As explained in the aforesaid Seeleyapplication, in the present detector system there need be no specialamplitude limiter stage employed prior to the detector circuit, sincethe detector itself is substantially immune to amplitude variations ofthe received FM signals. Hence, the I. F. amplifier tube 2 immediatelypreceding the detector circuit may possess normal and full gain, whichis the reverse of the usual operating condition for an amplitudelimiter.

The discriminator input circuit of the present FM detector comprises theshielded networks shown in dotted rectangles denoted by numerals 5 and 6respectively. The input coil 'I is indicated as part of the primarycircuit. The discriminator input network is utilized to provide theenergizing signal voltages for rectifiers D1 and D2. In general, it isdesired to employ a network constructed and arranged to derive from theangle modulated waves at coil l a pair of voltages whose relativeamplitudes vary in accordance with the angular deviations of the waveswith respect to a predetermined reference condition (whether phase orfrequency).

Considering the specic illustrative embodiment, coil 1 is shunted bycondenser 8 to provide a parallel resonant circuit tuned to theoperating I. F. The tertiary coil B is shunted by condenser I0. Theresonant tertiary circuit II, includuing coil 9 and condenser I0, istuned to substantially the resonant frequency of the primary circuit I2.Each of coils l and 9 is of the known inductance trimmer type.Specically, powdered iron cores or slugs are used for adjusting theinductance values of the respective coils 'I and 9.

The secondary resonant circuit I3 consists of coil I4 inductivelycoupled, as at M, to primary coil 7. The secondary coil I4 iseffectively shunted by its condenser I5 in series with a predeterminedintermediate section Ii of tertiary coil 9. The coil section I6 isprovided by tapping the secondary circuit leads Il and I8 to respective'spaced points I9 and 21B on coil 9. The secondary resonant circuit I3,composed of coil I4, condenser I5 and coil section IS, is tuned to theoperating I. F. value of 10.7 mc.

The primary resonant circuit I2 is arranged in circuit with anode 2| ofdriver tube 2. The connections of the I. Ffdriver stage are entirelyconventional, and need not be described in detail. The primary andsecondary circuits I2 and I3 are magnetically coupled, but they areshielded by any suitable shielding device vfrom electric or magneticcoupling with the tertiary circuit II. The shield 6 similarly shieldscircuit II from the primary and secondary circuits. Hence, the I. F.signal voltages are injected into the tertiary cir-` cuit II at coilsection I5. The resonant frequencies of the circuits I2, I3 and II areadjusted by respective powdered iron cores or slugs I4 and 9' of therespective inductances l. I4, and 9. Core I6 adjusts the electricalcenter of inductance il, and is located in proximity to coil section I6.

Before describing in detail the electrical relations existing betweenthe elements of the discriminator input system, the rectifier circuitswill be described. However, the description of these circuits will bequite general, since they are fully described in the aforesaid Seeleyapplication. Rectiers D1 and D2 are shown, by way of specic example, aselectron discharge devices of the diode type. It is to be clearlyunderstood that the diodes may have their electrodes embodied inseparate tube envelopes, instead of being embodied in one envelope as inthe GHG type tube. The cathode 28' of diode D1 is connected to the upperterminal, as diagrammatically Shown, of condenser IS and to the upperend of coil 9, whereas the anode 2l of diode D2 is connected to thelower terminal of condenser Ill and to the lower end of coil S. Theanode 22 of diode D1 and the cathode 23 of diode D2 are directlyconnected by condenser 24. The cathode 23 and the corresponding terminalof condenser 24 are established at ground potential for direct current.The magnitude of the condenser 24 is chosen so that the anode 22 ofdiode D1 is at ground potential With respect to modulation frequenciesi. e., audio frequency, as well as for I. F. Grounding this pointprovides a negative voltage at the anode of diode D1 which may be usedfor automatic gain control.

The anode 22 of diode D1 is connected to grounded cathode 23 by a pairof series-arranged condensers 25 and 2B. Each of these condensers 25 and2B has a relatively low impedance to I. F. currents, and they functionas I. F. bypass condensers. These condensers may, also, serve to providethe proper amount of de-emphasis in the audio signal, if they have theproper reactance to audio frequency currents. The secondary coil Hphasits low potential end or terminal connected by lead 21 to the junction33 of condensers 2.5 and 26. Hence, thev lower end or terminalofV Coil;I4 is at ground potential for I. E. currents, since the condenser 26connects it to ground.

It can be seen that the diodes D1 and D2 are arranged in reverserelation relativel to the connection in a conventional FM detectorcircuit of` the type employing balanced detector circuit diodes.l Thedetector circuit is completed by a resistor 2,8 shunted by condenser 24.The modulation Voltage, in this case the desired audio frequency`modulation signal voltage, is taken off by4 connecting an, audio outputlead toA the low I.- F. potential end of coil I4, i. e., the junctionCondenser,

point 33 of condensers 25 and 26. 29 is .an audio frequency couplingcondenser, and is inserted in series to ground with potentiometerresistor 39. The slider3l of the latter is adapted tov be connected tothe input grid of the` following, audio frequency amplifier tube (notshown). O f course, one or more audio ampliertubes may be. employed, andthe amplifier audio frequency signalsmay be reproduced in anysuitablemanner, asby a loudspeaker.

If no direct current path is connected from point 33 to point 32, or ifthe impedance of that pathis very high compared to the loadresistor 28,the direct current through the two diodes D1 and D2 is forced to beequal, regardless of whether the received FM signal is accurately tunedin or is oi resonance. This action provides substantial noise reduction.The present FM detector circuit has but a single direct cur. rent pathconnecting the diode rectiers inseriesaidingpolarity. Thus, resistor 28is included in such a path. The resistor isshunted by a condenser(capacity 24) which acts to inhibit changes. in thevoltage `dropacrossresistor 28 at a modulation frequency rate. This ratio detector isfound to be substantially immune to amplitude variationsy of theFMksignals at circuit I2.

There will nowV be explained the manner in which the discriminator inputnetwork of the detector functions. It is to bey understood, however,that the input network may bek used with inli'g.' 4lv isf. showngreatly. magnified, byvirtue of the fact that coil 9 yis completelyshielded from circuits I2 and I3.

Coil section I6 acts in the mannerrof a phase shifter element betweencircuits I3 and'II, and introduces a phase shift of 90 degrees in the I.F'. sign-alf voltage transmitted from circuit I3 to tertiary circuit I Ithrough the mutual inductance oi coil section- It.` Thisphase-shifted,or quadrature,voltage is applied in push-pull to the diode'sfDiandnDa.That is, the phase-shifted volt.

agesfappliedto each of diodes DrandDz are .in phase opposition relativeAto the4 electrical center pontilxe'd byadjustingcore Is'. Adjustment,

ofwthe latter fixes the electrical center of coil 9.

Thendirect input voltage to coil 9 is applied by direct connection atthetwo tapsISand 20 on coil 49 leading fromY the circuit I3; The totalSince themsecondary circuit I3,is

V20 is about 11/2 turns.

induciancein circuit I3. Y1512.011iprisci.ofnils Il. and I5 in series,Since coil I6 .(theminductance between taps I 9 and20 ofcoil 9) isnormally veryV small compared with coil I4', and alsoY with coil 9, thearrangement is substantially a direct v`conductive connection from thehigh potential end of circuit I3 to the mid-tap of coil 9; Since secr.tion I6 of coil 9 is common to both' circuits I3 and II, and theresonant currents of both; circuits I3 and II flow through thissection,it acts; as acommon couplinginductance for theH two. circuits. Thearrangement is similar toa normal. T configuration for common inductancecoupling.. of doube-tuned band pass filters. The voltages induced incircuitr II may be considered as sec-Y ondary voltages obtained byinductance coupling from circuit I3, the` primary as far as. circuit IIis concerned. Sinceboth circuits I3 and II are. tuned to the samefrequency, the normal phase relation exists` between the primaryV andYsecondary `voltages `at resonance.

Push-push voltages result, therefore,.frorn the direct electricalconnection between the. high potential end of circuit I3 and themid-tapvof the coil I6 in circuit I I as explained above. The push-pullvoltages result from the coupled, or induced, voltages in coil 9, andare obtained byv connections to the opposite ends of coil 9Y whenoperating as a secondary of a coupled circuit-as-Y explained above.

The Vspaced taps I9 and 20, as explained, provide the common couplinginductance between. circuits I3 and Il. Thek magnitude ofA theinductance I6 between the taps I9 and 29, relative to the magnitudes ofcoils I4 and 9, determines the degree of coupling between circuits I3and Il. The coefficient of coupling increases with increasing values ofcommon inductancey as in a T configuration for band passfilters. Thepercentage coupling between coils I and I4 may be about 5%. Thepercentage coupling between coils I4 and 9, provided by coil section I6,maybe about 2%.

In Fig. 2 there is shown a sectionthrough coil 9 showing coil 9 with thetaps and cores depicted in their relative positions. It vwill beV notedthat the taps do not occur at the physical center of the winding 9. Theyarelocated forexample 9% turns and 111/3 turns from thef two'y ends. Thepresence of the inductanceradjustfing core 9' in the short end of thewinding compensates this apparent unbalance. The core I6 is in alignmentwith core 9 within the coil from- F. The coil 9 is wound on the externalsurface of the form. The distance between taps I9 and Core I6 is longerthan coil section I6. Further, movement of core |67: does not aiect theinductance of section I 6 or its coupling to coil 9.

It will now be seen that each diode D1 andDz. has a pair of I. F. signalvoltages in phase quadrature applied thereto at resonance` It follows,then that the resultant I. F. signal voltages; applied to cathode 29 andanode 2l will be.. equal at the I. F. value, and the rectified voltageswill be of equal magnitude. If, at some laterinstant, the I. F; signalenergy has afrequency: different from the resonant frequency-ofcircuit'A II, there occurs arphaseshift of thegsignal en-g ergytransmitted through theA coilV section I6 which is greater or less than90 degrees depending on the direction and extent of frequency differencebetween the instantaneous. frequencyof the FM signals andthepredetermined res onant frequency `of the `tuned circuitsY I2', I3 andII. 'his means that there will be applied to the diodes Di and Deresultant signal voltages of dify ferent magnitudes, and, therefore, therectified voltages will be ofV diierent magnitudes.

It will be observed that my'invention makes it possible to use ironcores for adjusting the three tuned inductances '1, I4 and 9 of thediscriminator input circuit. Further, the auxiliary iron core I permits`adjustment of the electrical center` of thetapped third inductanoe 9thereby to securea symmetrical response while using a conventionaldouble-ended transformer construction. Core 9', located at one end ofthe winding 9, functions to adjust the value of the inductance of coil 9for correct tuning.

The cascaded circuits I2, I3 and Ii provide maximum sensitivity andselectivity. Since circuits I3 and II are inherently low QV circuitsunder normal operating conditions due to loading by the relatively lowvalue of the load resistor 23, the addition of another tuned circuit I2in the plate circuit of the driver tube 2 provides additionalselectivity. It, also, provides a means of obtaining a better impedancematch between the low Q circuit I3 and the driver tube plate impedance.It will be noted that generally circuit I3 serves as a coupling linkbetween the primary circuit i2 and the circuit II with signal transferoccurring from coil I to coil I4 by magnetic coupling, and from circuitI3 to circuit II by the common inductance coupling provided by thesection IB of coil 9. The coecient of coupling between coil i and Illand between coils I and 9 are chosen in combination` to provide bestsensitivity and linearity across the pass band.

It is pointed out that the junction point t3 between condensers and 25is connected to coil I4, rather than to coil l, to avoid the additionalphase shift between the tuned circuits I2 and I 3 which might otherwiseupset the quadrature phase relationship at the diodes. The fact that thedirect signal voltage is fed from circuit I3-to coil 9 at taps IS and 29might create some unbalance. However, since coil section IS is such asmallportion of coil 9, the unbalance is very small. In actual practiceno sign of unbalance is apparent. The` embodiment of the electricalcentei` adjustingrcore I permits compensation of production variations.

It would be possible to locate both cores 9' and It' at the two ends ofcoil 9. In that case both cores would be adjusted simultaneously forproper tuning and balance. of coil 9. This method of locating the coresis not as satisfactory as locating core I6 approximately in the centerof the winding, since both cores have equal effects on tuning ond theoperation of adjusting for both tuning and balance is moreY difcult toperform.

While the arrangement shown in Fig. I using spaced taps on coil 9 is thepreferred arrangement, there may be situations where the couplingbetween the secondary circuit I3 and tertiary circuit II is entirelycapacitative, rather than inductive. In this case the high potentialside of circuit I3 is connected by a single tap to the midpoint of coil9, and this need be the only change in the construction. Fig. 3 showssuch a circuit modification. The common impedance in this instance isthe diilerence in capacity to ground from cathode 2G and from anode 2|,which is the unbalan'ce to ground of thel two sides of tertiary winding9. This Aform of the circuit may be useful where very low coeiiicientsof coupling are desired, and it be'- cornes physically dicult to reducethe value of the coupling provided by coil section I6 to the propervalue.

In-Fig. 3 the circuit I3 has its coil I4 shunted by condenser I5. Thehigh potential lead I'I is connected to the midpoint I3 on coil 9. Thenumeral 4D denotes the common impedance, which is the imbalance toground of the two sides o winding 9. It will be noted that cores 9' andI6' provide, as in Fig. 1, respective adjustment of total inductance ofcoil 9 and the electrical centerof circuit II. In Fig. 3, therefore, thecommon impedance path between circuits I3 and II is obtained by thesingle tap I9 at the electrical center of the coil 9 'connected to thehigh potential junction of the second circuit inductance I4 andcapacitance I5. The capacitative unbalance'lll to ground is provided romthe opposing ends of inductance 9 causing a portion of the resonantcurrents of both circuit I3 and circuit II to flow through the commoncapacitive path provided by the unbalance. While I have indicated anddescribed a system for carrying my invention into effect, it will beapparent to one skilled in the art that my invention is by no meanslimited to the particular organization shown and described, but thatmany modications may be made without departing from the scope of myinvention.

What I claim is: 1. A frequency discriminator network comprising a rst,a second and a third resonant circuit rranged in cascade, said rstcircuit having sig nal input terminals for impressing thereon afrequency modulated wave, said second and third circuit including eachan inductor, a tuning capacitor connected tothe low potential end of thesecond circuit inductor, said third circuit having signal outputterminals at opposite sides thereof,v

said rst circuit being magnetically coupled to said second circuit,means electrically associated with the third circuit inductor toestablish an electrical center point thereon, two taps disposedapproximately symmetrically about said electrical center point, aconnection from one of said tapsito the high potential end of saidsecond circuit inductor, a connection from the other one of said taps tothe high potential end of the second circuit capacitor, thereby causingthe resonant currents of both said second and third circuits to flow inthe portion of said third circuit inductance included between said tapsand providing a common coupling impedance for said second and thirdcircuits, and said connections from said taps'to said second circuitalso providing a conductive connection between the high potential sideof said second circuit to said electrical center point.

2. A frequency discriminator network as defined in claim l wherein saidelectrical center establishing means consists of a paramagnetic coredisposed in said third circuit inductor and being appreciably shorterthan the total winding length of said third circuit inductor, wherebymovement of said core about the midpoint to acljust said electricalcenter produces no appreciable change in the total inductance of saidthird circuit and thus provides an independent ad-r justment, and saidcore being appreciably longer than the spacing between said taps andbeing nominally located approximately centrally within the spacingbetween said taps, whereby movement of said core to adjust saidelectrical center produces no appreciable change in the inductance ofthe portion of said third circuit inductv ance included between saidtaps and no appreciable change in coupling between said tapped portionand the total inductance of said third circuit.

3. A frequency discriminator network as defined in claim 2 wherein theresonant frequency of said third circuit is adjustable by a furtherparamagnetic core disposed in said third circuit inductor and adjacentto one end thereof.

DONALD MACKEY.

REFERENCES CITED The following references are of record in the le ofthis patent:

UNITED STATES PATENTS Number Name Date 1,751,996 Hansell Mar. 25, 19302,140,770 Schoeld Dec. 20, 1938 OTHER REFERENCES Carson et a1., NewFeatures in Broadcast Receiver Design, pp. 45-50, RCA Review, July 1937.(Copy in Div. 10.)

